The present invention relates to methods and apparatus for restoring the reduced light transmittance of an image-transmitting optical fiber bundle, which reduction results from X-ray or .gamma.-ray irradiation, and for thus making it once more usable for performing observations or examinations.
Fiber optic endoscopes are widely used for the purpose of observing or examining inaccessible cavities of a body which are impossible to observe or examine directly from the outside, and are generally divided into two categories, i.e., medical and industrial. Fiber optic endoscopes for medical use are used to observe or examine cavities of a human body such as the stomach, the duodenum, the colon, and the like; whilst the industrial are used to observe or examine the internal parts of machinery such as engines, nuclear reactors and the like. These fiber optic endoscopes comprise optical fiber bundles, one for transmitting an image of the internal parts to be observed or examined and the other for transmitting illumination from the outside. Each of said optical fiber bundles comprises an extremely large number of optical fibers with their opposite end portions cemented together and the major portion between the ends free to move relative to each other so as to be flexible, thereby to be insertable along a tortuous passage of a body.
Upon inserting a fiber optic endoscope into a human body, a fluoroscopic observation is often taken to locate accurately an inserted position of the top thereof relative to a region within the human body so as to ensure the safety of the person under examination. A certain fiber optic endoscope for medical use, for instance a duodenum endoscope, can be utilized for the purpose of endoscopic retrograde cholangiopancreatography (ERCP) examinations wherein a contrast medium is, for a fluoroscopic observation, injected into the pancreatic and bile ducts through a tube which is inserted in a therapeutic instrument guide channel of a fiber optic endoscope. As described above, fiber optic endoscopes for medical use have many uses in connection with fluoroscopic observation. As a result, an optical fiber bundle is frequently exposed to irradiation through a protective rubber tube.
Generally, the exposure of optical glass to X-ray or .gamma.-ray irradiation induces coloration thereof and thus decreases its light transmittance. According to published reports, the irradiation-induced coloration is due to so-called color centers. Irradiation interacts with the electrons of the atoms of the molecules of glass to release them. The released electrons impinge upon the electrons of other atoms to release them also. As a result, positive holes are formed having positive charges. Although a large number of released electrons are recombined with positive holes, the remaining released electrons are partially bound to structural imperfections of the optical glass to form such color centers. Because these electrons and positive holes at the color centers are weakly bound, glass absorbs light with a wavelength greater than the fundamental absorption band of the crystal before irradiation, to form another fundamental absorption band in the range of wavelengths of visible light.
The irradiation-induced coloration of glass is unavoidable in an image transmitting optical fiber bundle having thousands to tens of thousands of glass fibers each of which is composed of a fiber core and fiber cladding and has a diameter of the order of microns. An optical fiber bundle used in a fiber optic endoscope is frequently exposed to irradiation and thus suffers an increase in its absorption of light having a wavelength 400 to 550 nm, and so becomes colored yellowish brown. The yellowish-brown coloration appears in a fiber bundle within a protective tube after irradiation by a certain number of Roentgens (R) and deepens with further exposure. A fiber optic endoscope having an image-transmitting fiber bundle with such irradiation-induced coloration may be unacceptable for observing or examining an image therethrough and so may be returned to the manufacturer to replace the image-transmitting fiber bundle. But an image-transmitting fiber bundle is very expensive; and moreover, the replacement thereof is extremely complex and hence very costly.
To prevent optical glass from becoming colored yellow, it is known to mix cerium oxide in the composition of the glass. But since a cerium-oxide-containing glass has a tinge of yellow itself, this decreases its light transmittance. Optical elements such as optical lenses, prisms and the like, which are thin in the direction of the optical path, have substantially no effect on observations therethrough even if a decline of light transmittance is suffered; but a long optical fiber of diameter 10 to 50 microns and overall length 700 to 1200 mm is influenced considerably owing to marked degradation of the light passing therethrough. For this reason, image-transmitting glass fiber bundles conventionally used are made of glass without cerium oxide.
Another alternative for preventing irradiation-induced coloration of a glass fiber bundle is disclosed in Japanese Utility Model Publication No. 53-43025, in which an image-transmitting fiber bundle is inserted through a sheath of a concentric helix of metal strip covered with an irradiation-shielding layer of material such as lead, cerium or the like. This has the advantage that the sheath can prevent the image-transmitting fiber bundle from being exposed to excessive irradiation. However, coloration may still be induced by irradiation through gaps between the helical turns of the metal strip.
We obtained empirically the result that the fading of a colored image-transmitting fiber bundle induced by irradiation could be caused by visible light radiation, and then, the light transmittance thereof was recovered to a degree acceptable for performing observations and examinations.